The flexible polyurethane foam industry is being challenged by the EPA and OSHA to dramatically reduce the use of halo-carbon blowing agents, i.e., methylene chloride (MeCl2), as a process chemical and reduce employee exposure in the workplace. Environmental pressures, employee safety and ever-tightening governmental regulations have shifted the flexible slabstock polyurethane foam market away from the use of conventional blowing agents such as CFC-11, methylene chloride, 1,1,1-trichloroethane, and acetone. The industry responded by developing polyurethane foam made with higher-water based formulations. The physical blowing of such high-water polyurethane foam formulations occurs primarily from the carbon dioxide given off as a result of the reaction of water and isocyanate, and to a lesser extent from the vaporization of volatiles including water resulting from the reaction exotherm. This blowing replaces the traditional foam expansion derived from the volatilization of conventional blowing agents.
The function of halo-carbons such as MeCl2 in flexible polyurethane foam is twofold. Its purposes are primarily to contribute to the expansion of the reacting polymer, thereby producing a foam with a reduced density, and secondly, to remove the heat created by the exothermic nature of polyurethane reactions through its heat of vaporization.
Flexible polyurethane foam is the result of two reactions from three primary reactants, commonly a polyol, a multi-functional isocyanate, and a foaming agent, also called a blowing agent. The most common blowing agents are water and, typically, reaction products from the reaction of the isocyanate and water. The polyol and diisocyanate react to form urethane linkages, while the water and diisocyanate react to form urea and carbon dioxide (CO2). This combination of soft and hard segments form a polymer matrix that is expanded by the gaseous components. In water blown foams, CO2 is considered the primary blowing agent, while MeCl2 or any other physical blowing agent is considered an auxiliary blowing agent.
Auxiliary blowing agents are important for foam producers because they enable softer and lower density grades of foam that permit a wider product offering. The shift to these higher-water formulations and away from conventional blowing agents has placed many additional demands on flexible slabstock foam production. Producing these products without auxiliary blowing agents necessitates pushing the formulation chemistry to enhance the production of CO2. The use of higher amounts of water typically results in increased foam exotherms leading to increased foam discoloration, scorching problems and potential for fire. Also, an increased urea content is common in higher water systems, which results in higher hardness values.
This often results in foams with poor physical properties or unsafe exotherm values. A dramatic decrease in foam quality as evidenced by key physical properties of the foam such as compression sets, tensile strengths, tear strengths, and elongation values are also common in most conventional higher water systems.
Experiments with water-blown polyurethane (PU) foams that incorporate the plasticizers described in U.S. Pat. No. 5,624,968, the disclosure of which is incorporated by reference, showed some challenges exist for the product. Low densities can be reached with a combination of plasticizer and high water, but there is increased danger of fire as the level of water increases. One solution, besides adding methylene chloride to cool the foam and/or to replace some of the water, is to make low isocyanate index foams (meaning a foam having less than the stochiometric amount of isocyanate groups needed to react with all of the hydroxyl groups and water molecules present in the formulation). Low isocyanate index foams limit the potential for excess isocyanate groups to remain after the exothermic blowing and gelling reactions. These excess groups would otherwise react with atmospheric water vapor, extending the heat history of the foam. Also, the reaction of isocyanate with urethane and urea to form allophonates and biurets, respectively, contribute to the exotherm. In addition to the benefit of the reduction of fire danger, low isocyanate index foams allow lower firmness. Primarily, this is due to a decline in the amount of the hard polyurea segments produced since not all of the isocyanate reactive compounds, such as the water molecules in the formulation, will find isocyanate groups.
Unfortunately, however, it is the nature of low isocyanate index foams, whether or not they contain plasticizer, to have low integrity. Without intending to be bound to theory, an explanation is that not all of the hydroxyl groups on the polyol find isocyanate groups with which to bond. Allophanates and biurets increase PU foam integrity by increasing the crosslinked polymer matrix of the foam. Often during processing, foam with low indices (the isocyanate index, or index) has lower green strength that increases the likelihood of encountering splits, which are sizable openings within the foam. These foams frequently also have poor tear strength and tensile stress even after full cure.
The problems associated with low index foams limit the range of foam grades that can be made successfully with plasticizer alone. Low density and/or low firmness foams in all-water or reduced-methylene chloride formulations are nearly impossible to make due to integrity problems and processing difficulties.
One of the primary chemical solutions to have evolved to date is the use of low index formulation technologies, such as described in U.S. Pat. No. 4,950,694, which provides lower exotherms and lower hardness or indentation force deflection values relative to conventional index, all-water-based systems. With such low index systems, lower load foam grades can be produced without the environmentally harmful conventional and auxiliary blowing agents. These foams, however, have unacceptable commercial characteristics including poor tear strength, splitting, poor recovery properties, and poor hand.
Thus, some softer foam grades are not attainable using only water as the sole blowing agent. These higher water systems also typically are more difficult to process than the conventional lower water counterparts. These and related problems have generated several solutions to overcome the inherent pitfalls of current all-water-blown slabstock foam production technology.
U.S. Pat. No. 5,539,011 discusses an amine polyisocyanate catalyst additive that is useful in softening all-water blown flexible polyurethane foams that have commercially necessary properties. The additives are tertiary amines which contain at least one contiguous three carbon chain and should be added to the foam system at about 0.1 to 2.0 parts per hundred of polyol. The flexible PU foams described contain certain tertiary amine polyisocyanate catalysts where water is the blowing agent (and optionally using other conventional blowing agents), with an isocyanate index of 60-120, preferably 80-115, and most preferably 85-95. Furthermore, the '011 patent mentions that the foaming process can optionally be conducted in the presence of a combination of a cell opening agent, i.e. a polyethylene oxide monol and/or polyol with equivalent weight >200, and a crosslinking/extending agent, preferably a low equivalent weight (<200) polyfunctional glycol.
U.S. Pat. No. 4,950,694 describes flexible polyurethane foams prepared using water as blowing agent, and optionally other blowing agents, and a low equivalent weight, i.e., less than 200, crosslinking or chain extending agent with an isocyanate index of 60-95. Polyfunctional glycolamines, i.e. diethanolamine, are the preferred crosslinking/extending agents. The '694 patent mentions that the problem of foam splitting at low isocyanate indicies can be treated by using particular crosslinking/extending agents without introducing unacceptable tightness to the foam, and that a cell opening agent, i.e. a polyethylene oxide monol and/or polyol, is preferably incorporated.
U.S. Pat. No. 4,288,566 describes flexible PU foams prepared using water as blowing agent, and optionally other organic blowing agents, and a crosslinking agent. The isocyanate index is not directly disclosed but an excess of isocyanate is apparently used. The crosslinking agents disclosed include the ethylene glycol, glycerine, erythritol, and sugar alcohols, e.g. xylitol, sorbitol and mannitol. The '566 patent also mentions that low molecular weight, i.e., molecular weight between 32 and 400, glycols or other polyols, can be used.
U.S. Pat. No. 4,211,849 describes flexible PU foams prepared using water and/or organic propellant as blowing agent and a crosslinking agent which is a crystalline polyhydroxy compound having at least three hydroxy groups. The '849 patent discloses crosslinkers which work in the invention as well as some which do not work. The '849 patent teaches against using aromatic polyhydroxy compounds as crosslinkers.
U.S. Pat. No. 5,624,968 discloses a flexible plasticized PU foam in which water or a non-halogen gas is used as the foaming agent in conjunction with a plasticizer selected from phosphate esters, phthalates, or benzoates. The '968 patent also discloses polyol chain extenders can be used. However, the chain extenders listed therein not particularly effective, and are better suited in non-foamed PU cast elastomer systems. Specifically, all of the chain extenders listed would not retard processing at higher isocyanate indicies, but they would not work to produce commercially acceptable flexible foams at lower isocyanate indicies.
In Japanese patent application 59-226034, published Dec. 19, 1984, a method for manufacturing a foamed urethane molded article is described. A urethane foam is produced from the reaction of a polyol and an isocyanate compound using both a fluorocarbon compound and a small amount of water as the foaming agents. Both toluene diisocyanate and p,p′-diphenylmethane diisocyanate were used as the isocyanate compound. A phthalic acid plasticizer, such as the specifically discussed di-2-ethylhexyl phthalate, butyl benzyl phthalate, and dibutyl phthalate, was added to the reaction to reduce the lower mold temperature required for molding the desired articles. This Japanese application teaches the production of rigid foams, as shown by the sphere penetration test; and the plasticizers are added to reduce the required mold temperature.
What is needed in the art is a PU formulation that provides soft foams made with little or no auxiliary blowing agents but having commercially acceptable characteristics.